1. Field of the Invention
This invention relates to liquid metal purification and more particularly to a device for and method of removing metal oxides from liquid alkali metals. It specifically relates to a sodium oxide trap wherein sodium oxide is precipitated and removed from a stream of liquid sodium.
2. Prior Art
Liquid metals, particularly the liquid alkali metals, have found wide use as heat transfer media. For example, sodium and mixtures of sodium and potassium are used as a heat transfer medium for use in conjunction with a central solar receiver. A problem encountered with the use of liquid alkali metals as heat transfer media is the formation of oxides by contamination with atmospheric oxygen or reaction with objects in contact with the liquid alkali metal. These oxides must be removed since they contribute to accelerated corrosion of the materials of construction. In addition, they tend to obstruct flow passages.
Heretofore the conventional device for removing, for example, sodium oxides from liquid sodium, was a cold trap which made use of the fact that the solubility of the oxide is proportional to temperature. The oxides are removed by lowering the temperature of a stream of liquid alkali metal to a temperature below the precipitation point of the liquid metal oxide but above the melting point of the liquid alkali metal. The metal oxides are then removed by filtration or entrapment with a metal mesh. The generally accepted basis for cold trap design has been to pack the trap with mesh. The mesh designs require careful control during operation to minimize the oxide concentration differential as well as the temperature differential across the mesh to prevent an excessive precipitation rate which could result in premature plugging of the mesh. In spite of the fact that many studies have been conducted in the United States and abroad in an effort to improve the design of cold traps and the understanding of their working, the use of a mesh has remained a key feature of cold trap design.
U.S. Pat. No. 3,558,122 discloses a mesh-type of liquid metal purifier. The device comprises a metal oxide collector, such as a mesh, to be immersed in a body of liquid metal, a heat exchanger outside the body of liquid metal, means for transferring heat from the metal oxide collector to the heat exchanger, and a moveable thermal barrier positioned around the metal oxide collector.
U.S. Pat. No. 3,693,959 shows yet another variation of a mesh-type cold trap for liquid metal. The device comprises a vessel and a cylindrical duct member arranged concentrically in the vessel. A flow of liquid metal from an inlet is directed into the vessel downwardly along the annular interspace defined between the duct member and the inner wall of the vessel, and then into the lower end of the vessel and upwardly through the duct member. The upper length of the duct member and liquid metal inlet end of the vessel is of a thermally conducting nature while the remaining length of the duct member towards the lower end of the vessel is of double wall construction so as to be of a thermally insulating nature. The duct member is provided with a filter to remove any precipitated particles which are carried over from the bottom of the vessel.
U.S. Pat. No. 3,831,912 discloses yet another form of cold trap utilizing a mesh. The mesh comprises a net-like member constructed of large meshes which are knitted with a plurality of fine wires and have a plurality of piled or stacked spaces between one net face and another net face opposing the first.
U.S. Pat. No. 3,618,770 discloses yet another type of cold trap utilizing a mesh. Patentees disclose that the effectiveness of a cold trap in nucleating and precipitating oxide impurities is increased by electromagnetic stirring of the cooled sodium. The electromagnetic stirring action is accomplished with a polyphase rotating magnetic field.
The disadvantage of all mesh-type traps is, of course, that the upstream surface of the mesh tends to become coated with the oxide precipitate, eventually plugging and blocking the flow. Obviously, therefore, there is need for an improved cold trap which would not require a mesh for efficient oxide removal.